In recent years, there has been an increasing demand for crystalline thermoplastic, heat-resistant and easily melt-processable resins in automotive, electronic and electric industries. For instance, with the spread of electronic ranges, electrical ovens and electronic oven/range combinations in domestic electrical fields, heat-resistant and electromagnetic wave-transmitting containers for foodstuffs have been much in demand. For that reason, attention has now been paid to easy-to-crystallize polyethylene terephthalate (hereinafter abbreviated as "PET") as a material capable of molding or forming heat-resistant plastic containers, e.g., thermoformed food containers which may be used with domestic electronic ranges, electrical ovens, etc. and can stand up to an oven temperature close to 200.degree. C.
However, PET containers may be used in applications where relatively short-time heating meets the purpose, as is the case with electronic ranges, but are disadvantageous in that they are still so poor in heat resistance that they cannot withstand a high temperature of 200.degree. C. or higher, inter alia, 220.degree. to 230.degree. C., in electronic ranges or electrical ovens.
Thermosetting resins such as unsaturated polyester resins or epoxy resins are moldable by the sheet molding compound (SMC) or bulk molding compound (BMC) system, and may be molded into food containers. The SMC is a sheet-like intermediate material for press molding obtained by impregnating in matted glass fibers a resin paste prepared by mixing a thermosetting resin with a filler, catalyst (a curing initiator), releasing agent, chemical thickener and the like. This sheet-like material is stored at a given temperature for a given period of time, and is designed to be molded or formed by a press at the time when the resin is semi-set or dried to the touch under the action of the thickener. The BMC is prepared by kneading together with glass fibers (chopped strands) a resin paste wherein a thermosetting resin is blended with a filler, chemical thickener, catalyst (a curing initiator), pigment, releasing agent and the like. The BMC may be molded by press molding, transfer molding, injection molding and the like. Heat-resistant food containers may be produced from thermosetting resins by such molding processes. However, food containers formed of thermosetting resins such as unsaturated polyesters are disadvantageous in that they cannot stand up to high temperatures prevailing in electronic ranges or electrical ovens due to their heat-resistant temperature being as low as about 210.degree. C.
On the other hand, engineering plastics such as poly(arylene sulfide) (PAS) and poly(arylene thioether-ketone) (PTK) are now being developed or in practical use as thermoplastic resins excellent in heat resistance.
PAS is a thermoplastic resin used in wide fields as the engineering plastics showing excellent resistance to heat, chemicals (acids, alkalis, solvents), oils and hot water, good processability and having excellent mechanical properties. Taking advantage of its excellent properties, PAS is also used in the form of films or fibers. PAS is a crystalline resin, and is better in heat resistance than PET due to its melting point higher than that of PET. PAS is also excellent in the resistance to wet heat, solvents, etc.
There are some disclosures on PTK, for instance, German Offenlegungsschrift 34 05 523 A1, Japanese Patent Laid-Open Publication Nos. 58435/1985, 104126/1985 and 13347/1972, Indian J. Chem., 21A, pp. 501-502 (May, 1982), and Japanese Patent Laid-Open Publication No. 221229/1986. With the conventional PTK described in such publications, however, difficulty was encountered in melt molding by conventional melt molding/forming processes such as injection molding or extrusion, since it was so poor in melt stability that it lost its crystallinity or underwent a curing reaction with increases in melting viscosity during melt processing.
However, it has been found that PTK improved highly over the conventional PTK in melt stability are obtained by modifying the polymerization procedures, i.e., carrying out polymerization without adding any polymerization aid while taking into consideration the selection of a charge ratio of monomers, a shorter polymerization time at high temperatures, the selection of the material of the reactor used and the like and, optionally, conducting a stabilization treatment in a final stage of the polymerization. Such PTK is melt stable and moldable or formable by conventional melt processing (Japanese Patent Application No. 62-118619).
Incidentally, U.S. Pat. No. 4,690,972 specification discloses that PTK is added to PAS as a nucleating agent. However, said PTK is different from the said melt-stable PTK, and nowhere in that specification are heat-resistant molded or formed articles such as heat-resistant food containers disclosed.
The present inventors have made intensive studies so as to obtain heat-resistant compositions and molded or formed articles, taking advantage of the properties of PAS and melt-stable PTK as mentioned above.
In consequence, it has been found that a heat-resistant container is obtained by melt-molding or forming the melt-stable PTK alone or a thermoplastic material obtained by mixing 100 parts by weight of a thermoplastic resin such as PAS with said PTK in an amount of 25 parts by weight or more (Japanese Patent Application No. 195806/1987). This approach has however been found to involve problems such that a high melting-point PTK or a PTK-PAS composition containing the PTK in a large proportion requires a high-temperature mold in order to obtain a molded article excellent in surface characteristics and the resultant molded article is expensive due to the abundant use of the PTK more expensive compared to PAS.